1. Field of the Invention
The present invention relates to a cutting tool of a polycrystalline hard sintered body containing a cubic crystal boron nitride(hereinafter, referred to as CBN) and having a cutting edge treated at an insert thereof.
2. Description of the Related Art
A material where powder of CBN is sintered with various kinds of bonding agents exhibits an excellent performance for cutting iron family metals or cast iron having high hardness. Particularly, in a machining of hardened steel having high hardness, when the machining is carried out with a tool of CBN, a roughness on a finished surface and a precision in machined size are equivalent to those of a grinding. Accordingly, the grinding using a conventional grinder is changed to a cutting using the tool of CBN in a part of the cutting of hardened steels.
This inclination has more developed, and such a demand has recently been raised that the cutting is substituted for a machining field requiring the high precision only possible conventionally with the grinding. Objects to be cut by a tool containing CBN are materials of hard quality such as hardened steel or cast iron. It has been hardly conceived to obtain a finish surface near to a mirror surface by the cutting. A precision machining referred to herein is meant by a dimensional precision such as the surface roughness and the roundness on the cut surface.
On the other hand, the most advanced precision cutting is a cutting of non-ferrous metal using single crystal diamond. A main application field is in a machining of aluminum based alloys such as a base board of memory disk or polygon mirror. There is nowadays a tendency to replace a single crystal with polycrystal diamond easier to machine at lower cost. In the polycrystal diamond tool, if a large unevenness is formed on a ridge of the cutting edge due to difference in steps at crystal interface, it cannot be removed even by grinding the flank or the rake face of the tool. There occur problems that this unevenness remains in the ground surface, and is transferred to a work piece to hinder the machining at high precision. A case of polycrystal diamond cutting tool described in Japanese Patent Unexamined Publication No. Hei. 6-190610(JP-A-6-190610) has solved this problem by chamfering the tool at an insert thereof. That is, the flank face of the cutting edge using the polycrystal diamond is formed to be a ground face without ground trace, and the ridge of the cutting edge is chamfered with a fine width by the grinding so as to smoothen the cutting edge at the ridge. However, since in the tool of polycrystal diamond, carbon composing diamond reacts with steel, this tool cannot be used to the cutting of a hardened steel or a cast iron.
In view of the problems involved with the conventional technique, when a hardened steel is subjected to the cutting, the invention is to make the surface roughness on the cut face 1.6 xcexcm or less at Rz and to improve the dimensional precision of the roundness to be 3 xcexcm or less. As a field of uses, many cases are of materials to be cut of diameter being around 30 mm or smaller. Namely, the superior roughness of a finished surface and the high precision in machined size are realized by improving a shape of the cutting edge and the surface roughness of the cutting tool of a polycrystalline hard sintered body containing CBN. xe2x80x9cRzxe2x80x9d is meant by the average roughness of the ten point system specified by B0601 xe2x80x9cSurface roughnessxe2x80x94Definition and Designationxe2x80x9d of JIS.
According to a first aspect of the invention, the cutting tool of the polycrystalline hard sintered body having a cutting edge treated at an insert according to the invention comprises the polycrvstalline hard sintered material containing 20 vol % or more CBN, radius of curvature in cross section of a ridge of a cutting edge is between 5 xcexcm or more and 30 xcexcm or less, a flank and a rake face or negative land of the tool are smoothly continued at said radius of curvature in cross section. And, surface roughness of the ridge of the cutting edge is from 0.1 xcexcm or more to 1.0 xcexcm or less according to an average roughness of the ten point system (Rz). By making such a structure, it is possible to machine a work piece at high precision.
According to a second aspect of the invention, the surface roughness on the rake face or the negative land of the tool ranges from 0.1 xcexcm or more to 0.5 xcexcm or less according to the average roughness (Rz) of the ten point system. By making such a structure, it is possible to offer a cutting tool of a long life, because the surface roughness on the cutting edge can be kept small, even if the ridge of the cutting edge is worn and moves backward during machining.
According to a third aspect of the invention, a wedge angle at the insert made between the flank and the rake face of the tool or the flank and the negative land of the tool is from 65xc2x0 or more to 125xc2x0 or less. By making such angles, it is possible to economically make the cutting edge of the tool.
According to a fourth aspect of the invention, the polycrystal hard sintered material contains 20 vol % or more CBN, and the average grain diameter is from 0.01 xcexcm or more to 5 xcexcm or less. By making such a structure, it is possible to economically make the cutting edge of the tool.
According to a fifth aspect of the invention, the polycrystalline hard sintered material is bonded to a base material of the tool made of cemented carbide. Because, for carrying out the high precision machining, the polycrystalline hard sintered material must be bonded to a base material of the tool having high rigidity.
According to a sixth aspect of the invention, a pair of straight cutting edges respectively have length ranging from 0.2 to 0.6 mm, said cutting edges being connected to a nose r from respective terminals of the nose r of the polycrystalline hard sintered material toward a side of an indexable insert, and angle between a straight line bisecting said nose r and the straight cutting edges is formed to be 42xc2x0xe2x89xa6xcex810, xcex811 xe2x89xa645xc2x0. Thereby, the part of the straight cutting edge can drag the cut face to improve the surface roughness on the finished face.
According to a seventh aspect of the invention, the surface is formed with the coated layer by a chemical vapor deposition or a physical vapor deposition. The wear at the ridge of the cutting edge is reduced thereby, so that the roughness on the finished surface of the work piece may be improved, and the life of the tool may be lengthened.